1. Field of the Invention
The present invention relates to a fixing apparatus and an image forming apparatus that uses the same. The fixing apparatus is used in image forming apparatuses such as copiers, facsimile machines, and printers that employ an electro-photography or electro-static recording method. Especially, the present invention is related to the fixing apparatus that heats and fixes an unfixed image on recording material using an electromagnetic-induction heating method.
2. Description of Related Art
In recent years, there have been many researches performed on employing an electromagnetic-induction heating method for fixing apparatuses used in apparatuses such as copiers, facsimile machines, and printers. In such a fixing apparatus that employs the electromagnetic-induction heating method, an alternating current is applied to an exciting coil around which an alternating magnetic flux is generated. When the generated alternating magnetic flux permeates through a conductor, an eddy current (EC) is generated. Heat in the conductor caused by the EC is used for fixing an un-fixed image.
At the same time, many attempts have been made to shorten a warm-up period of the fixing apparatus, by decreasing, as much as possible, heat capacity for the heated portion of the fixing apparatus, and by strengthening thermal insulation. However, there are shortcomings, caused by decreasing the heat capacity of the heating unit and strengthening the thermal insulation, that the heat does not properly transfer in the width direction. Especially when narrow-width recording material is continuously fixed, temperature outside the recording material width abnormally rises, thereby causing hot offset, damaging and lowering life of a rubber member. In Related Art 1, as shown in FIG. 14, an attempt has been made to provide arch shaped conductive member 45a as a means for preventing excessive temperature of the heating roller 44 rising outside the recording material width. Conductive member 45a is located inside heating roller 44, which is heated with exciting coil 23, and directly facing the inner surface of heating roller 44.
[Related Art 1] Japanese Patent Laid-Open Publication 2001-125407
Nonetheless, conductive member 45a for shielding the magnetic flux has an arch shape facing to the inner surface of heating roller 44, and is disposed proximate to heating roller 44. Accordingly, during the warm-up period, the temperature rises as heating roller 44 is heated, and when a difference in temperature increases between heating roller 44 and conductive member 45a, conductive member 45a also raises its temperature as it receives radiated heat from heating roller 44 and heat conductance through air. At this time, conductive member 45a has an arch shape and has a large surface that faces to the inner surface of heating roller 44. Since conductive member 45a has a relatively large shape and large heat capacity, there is a high heat transfer from heating roller 44. Therefore, when the temperature of heating roller 44 and fixing belt 20 exceeds approximately 150° C., the heat rising speed slows down and the warm-up period thus becomes long. When the heat capacity is large, the heat in heating roller 44 continues to escape to conductive member 45a even after the temperature rises to the fixing temperature. Therefore, when the fixing of the recording material is continuously performed right after the temperature rise, quantity of heat to be provided becomes insufficient because the quantity of heat escapes to not only conductive member 45a but also the recording material, thereby lowering the temperature of heating roller 44 and fixing belt 20. This phenomenon is seen especially when the environmental temperature is low and thick recording paper is used for fixing. The temperature drop causes poor fixing quality. In order to prevent this problem, it is necessary to wait, prior to start fixing, for the temperature of conductive. member 45a to rise to a predetermined temperature. As a result, the warm-up period becomes long.